Diffuser, in particular for an axial flow machine

ABSTRACT

A diffuser for an axial flow machine has a transition from a ring channel having a first cross sectional area into an outlet space with a second, larger cross sectional area along a machine axis of the axial flow machine. The transition includes a plurality of steps.

CROSS-REFERENCE TO PRIOR APPLICATIONS

Priority is claimed to German Patent Application No. DE 10 2011 118 735.2, filed on Nov. 17, 2011, the entire disclosure of which is hereby incorporated by reference herein.

FIELD

The present invention relates to the field of axial flow machines and more particularly to a diffuser.

BACKGROUND

Diffusers which are arranged at the outlet of stationary gas turbines and which are to reduce the speed of flow of the gases coming out of the turbine and to bring about a build-up of pressure in order to improve the efficiency of the gas turbine, have been known for a long time in the prior art (see, for example, document EP 0 491 966 A1 or document US 2011/058939 A1 along with the attached FIG. 1).

In the past, various proposals have been made in order to improve the action of the diffuser at the outlet of a gas turbine and consequently the overall efficiency of the machine. Thus, among other things, document EP 0 265 633 B1 proposes dividing the diffuser into several part diffusers in the radial direction by means of flow-conducting baffle plates.

In US 2011/058939 A1, already mentioned, to improve the flow conditions in the diffuser the inner tapering part of the diffuser is provided with a controllable Coanda flow by way of which the flow in the diffuser can be influenced in a favorable manner. The inner part of the diffuser, the hub, tapers downstream without forming a step. From an external source, a gas is guided toward a ring chamber in the hub and from there is injected by means of a number of slotted nozzles in the direction of flow of the hot exhaust gases parallel to the surface of the hub. As a result of the known Coanda effect, said additional gas flow sucks in hot exhaust gas and deflects it in the direction of the hub. The exhaust gas flow is accelerated there and adapts to the surface of the hub which tapers downstream. In order to achieve a desired influencing of the exhaust gas flow in the diffuser, up to 4% of the exhaust gas mass flow in additional gas has to be injected, which is equal to not insignificant expenditure.

Contrary to this, EP 0 265 633 B1 provides a sudden transition in the cross sectional area at the outlet of the diffuser which is designated as a Carnot diffuser.

Although said measures provide certain improvements in efficiency, the possibilities for exerting influence in the region of the diffuser have not been exhausted by a long way.

SUMMARY

In an embodiment, the present invention provides a diffuser for an axial flow machine. The diffuser has a transition from a ring channel having a first cross sectional area into an outlet space with a second, larger cross sectional area along a machine axis of the axial flow machine. The transition includes a plurality of steps.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in even greater detail below based on the exemplary figures. The invention is not limited to the exemplary embodiments. All features described and/or illustrated herein can be used alone or combined in different combinations in embodiments of the invention. The features and advantages of various embodiments of the present invention will become apparent by reading the following detailed description with reference to the attached drawings which illustrate the following:

FIG. 1 shows the schematic design of a gas turbine with an exhaust gas diffuser, as is known;

FIG. 2 shows the inside design of a conventional Carnot diffuser;

FIG. 3 shows, in comparison to FIG. 2, the inside design of a multi-step diffuser according to one exemplary embodiment of the invention;

FIG. 4 shows a perspective side view of a 2-step diffuser according to another exemplary embodiment of the invention; and

FIG. 5 shows the inside design of a 2-step diffuser with Coanda control according to a further exemplary embodiment of the invention.

DETAILED DESCRIPTION

In an embodiment, the invention provides a diffuser, in particular for an industrial gas turbine, which results, in a simple manner, in a further improvement in the overall efficiency of the gas turbine.

An embodiment of the invention proceeds from a diffuser, in particular for an axial flow machine, preferably a stationary gas turbine, which diffuser transforms from a ring channel with a first cross sectional area into an outlet space with a second, larger cross sectional area along a machine axis. It is distinguished in that the transition is effected in several steps.

A first development of an embodiment of the invention provides that the cross sectional area inside the diffuser is increased in two steps. Said diffuser is designed in a particularly simple manner.

As provided in another development of an embodiment of the invention, the diffuser is realized as a Carnot diffuser.

A further development of an embodiment of the invention is distinguished in that the diffuser includes an outer casing and an inner casing, between which the medium flows through the diffuser, and that the steps are generated in the cross sectional area by diameter steps on the inner casing.

Another development of an embodiment of the invention is characterized in that a ring-shaped, convexly curved guiding surface which tapers in diameter is arranged between two adjacent steps, and that on the upstream step of the two steps there is provided an annular passage, through which a gas flow is able to escape and to flow along the guiding surface in the form of a Coanda flow. As a result, the flow in the diffuser is able to be influenced in a favorable manner.

The guiding surface is preferably arranged between the penultimate and the last step of the diffuser.

Yet another development of an embodiment of the invention is characterized in that the diffuser is arranged at the outlet of an industrial gas turbine.

FIG. 1 shows the schematic design of a gas turbine with an exhaust gas diffuser, as is known in the prior art. The gas turbine 10 shown in FIG. 1 includes a compressor 12, which sucks in air by means of an air inlet 11 and compresses it. The compressed air is supplied to a combustion chamber 13 and there is used for the combustion of a fuel 14. The resultant hot gas is expanded in a turbine 15 downstream under operating conditions and then flows through a diffuser 16 in order to slow down the speed of flow and to bring about a build-up of pressure.

FIG. 2 shows a highly simplified representation of the inside design of a conventional Carnot diffuser. In this case, the diffuser 16, which is realized in a concentric manner with respect to a machine axis 31, on the inlet side includes a ring channel 17, by means of which the exhaust gas 19 of the turbine flows into the diffuser 16. Connecting to the ring channel 17 with its comparatively small cross sectional area is an outlet space 21, the cross sectional area of which is substantially larger for the flow. The transition between the ring channel 17 and the outlet space 21 is effected, in this example, by means of a sudden step 22, which characterizes the diffuser 16 as a Carnot diffuser. Radial struts 18, which connect the inside part and the outside part of the diffuser 16 and at the same time serve for steering the flow, can be arranged in the ring channel 17.

In contrast, the invention now proposes, according to the exemplary embodiment shown in FIG. 3, to realize the transition between the ring channel 17 and the outlet space 21 in multiple steps in the case of a diffuser 20. In the example shown, two steps 22 a and 22 b are provided for this purpose. A further step 22 c (shown by the broken line in FIG. 3) is optional. The number of steps, however, is not limited upward. The diameter jumps connected to the steps 22 a-c are limited in the exemplary embodiment in FIG. 3 to the inside part of the diffuser 20. However, it is also just as conceivable to provide diameter jumps on the outside part of the diffuser.

Such a multiply stepped inside contour produces a gain in the build-up of pressure which can be 0.1% of the turbine efficiency and in the case of a GT26 model gas turbine of the Applicant signifies a gain in capacity of almost half a megawatt.

In practice, a corresponding diffuser looks, for example, as shown in FIG. 4. The diffuser 20 a of FIG. 4 includes a ring-shaped outer casing 23 which surrounds an inner casing 24 in a concentric manner and together with the inner casing 24 defines a flow channel. The inner casing 24 and the outer casing 23 are connected by means of radial struts 25. Two rings 26 and 27, which are stepped in diameter and by means of which the multiply stepped expansion of the diffuser 20 a is brought about, are arranged one behind the other in the axial direction at the outlet of the diffuser 20 a.

In addition to the multi-stepped expansion of the cross sectional flow, the flow conditions in the diffuser can be influenced by means of a Coanda flow, as has been proposed, in principle, in document US 2011/058939 A1 mentioned in the introduction. To this end, according to FIG. 5, a ring-shaped, convexly curved guiding surface 28 which tapers in diameter is arranged between two steps 22 a and 22 b in the case of a diffuser 20 b. On the upstream step of the two steps 22 a and 22 b is provided an annular passage 29, through which a gas flow is able to escape and to flow along the guiding surface 28 in the form of a Coanda flow 30. In this case, the gas feed for the Coanda flow 30 can be effected in different ways. Contrary to what the aforementioned document teaches, however, as claimed in the invention an external reference source for an actively injected additional gas is to be omitted. By arranging the components in a proper manner, the pressure conditions prevailing in the region of the erratic cross sectional expansion of the diffuser are to be utilized in such a manner that, during operation, a wall flow 30 is automatically built up along the curved guiding surface 28 and said wall flow deflects the parallel exhaust gas flow 19. The static pressure p₂ behind the ring body 27 is higher than the inlet pressure p₁ at the annular passage on account of the deceleration of the flow through the cross sectional expansion. Accordingly, a flow 32 is formed from the higher pressure region into the lower one.

If more than two steps are present in the diffuser, the Coanda flow is preferably inserted between the penultimate and the last step.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. It will be understood that changes and modifications may be made by those of ordinary skill within the scope of the following claims. In particular, the present invention covers further embodiments with any combination of features from different embodiments described above and below.

LIST OF REFERENCES

-   10 Gas turbine -   11 Air inlet -   12 Compressor -   13 Combustion chamber -   14 Fuel -   15 Turbine -   16,20 Diffuser -   17 Ring channel -   18,25 Strut -   19 Exhaust gas -   20 a,b Diffuser -   21 Outlet space -   22 a-c Step (cross sectional area) -   23 Outer casing -   24 Inner casing -   26,27 Ring -   28 Guiding surface (curved convexly) -   29 Annular passage -   30 Coanda flow -   31 Machine axis -   32 Return flow 

What is claimed is:
 1. A diffuser for an axial flow machine comprising a transition from a ring channel having a first cross sectional area into an outlet space with a second, larger cross sectional area along a machine axis of the axial flow machine, wherein the transition includes a plurality of steps.
 2. The diffuser as recited in claim 1, wherein the axial flow machine is a stationary gas turbine, the diffuser being disposed in the stationary gas turbine.
 3. The diffuser as recited in claim 2, wherein the diffuser is disposed at an outlet of the stationary gas turbine.
 4. The diffuser as recited in claim 1, wherein the plurality of steps is two steps, the two steps increasing the cross sectional area inside the diffuser from the first to the second cross sectional area.
 5. The diffuser as recited in claim 1, wherein the diffuser is configured as a Carnot diffuser.
 6. The diffuser as recited in claim 1, wherein the diffuser includes an outer casing and an inner casing configured to allow a medium to flow therebetween during operation, the plurality of steps being formed by steps in a diameter of the inner casing.
 7. The diffuser as recited in claim 1, further comprising a ring-shaped, convexly curved guiding surface which tapers in diameter disposed between two adjacent steps of the plurality of step, and an annular passage disposed at an upstream step of the two adjacent steps configured to allow a gas flow to emerge and flow along the guiding surface in the form of a Coanda flow.
 8. The diffuser as recited in claim 7, wherein the two adjacent steps are a penultimate step and a last step of the diffuser. 